Combustion non-uniformity adversely impacts the efficiency, lifetime, and emissions of a combustor and associated machinery. For example, in turbine engines, hot spots at the exit of the combustor limit the operating temperature of the combustor and cause damage to the downstream turbo-machinery. Combustors usually use passive design features, such a large mixing regions, or lean-premixed operation to achieve combustion uniformity, but these approaches have their limits in terms of size, weight, and performance. Even with passive methods, non-uniformities persist. Active control measures can potentially increase combustion uniformity while relaxing the design and performance constraints of passive methods.
An active control system is dependent on a sensor that can sense the state of combustion in the hot flow-path of the combustor and the power-extraction machinery immediately downstream of the combustor. Passive optical sensors can measure the flow-path conditions by collecting emission from the hot gases and particles in the flow path. Spectrally selective detection can yield information on the physical properties of the flow-path such as temperature and product distributions. See, for example, U.S. Pat. No. 6,640,199 and U.S. Pat. No. 6,646,265. These approaches are generally applicable for monitoring the properties at the exit of the combustor, however, optical access to the combustor exit is limited, and it is often desirable to place the sensor with the fuel nozzle at the front end of the combustor. In pre-mixed combustors where the fuel/air ratio is uniform throughout the combustion region, a front-end sensor can collect radiation characteristic of the entire combustor. However, in non-premixed combustors, the sensor must view through multiple combustion zones each with very different fuel/air ratios and optical emission signatures.
Most combustors, such as rich-quench-lean (RQL) combustors, have multiple zones with different fuel/air ratios. The combustion mixture starts out relatively fuel-rich near the fuel injection point, and then becomes successively leaner as the fuel-rich mixture mixes with additional air. The characteristic emission of each combustion zone is different. Emission from the rich zones is dominated by emission from soot and short-lived intermediate species. Emission from down-stream, well-mixed areas is dominated by product emission. See: Yamaguci, T., K. T. V. Grattan, H. Uchiyama, and T. Yamada, “A Practical Fiber Optic Air-Ratio Sensor Operating By Flame Color Detection,” Review of Scientific Instruments, 68(1): 197 (1997); Docquier, N., Belhafaoui, S., Laca, F., Darabiha, N., Folon, J-C, Proc. Combust. Inst. 2000, 28: 431-8; and U.S. Pat. No. 7,334,413 teach control using a sensor placed at or near the fuel nozzle to monitor the emissions from the active flame front in premixed natural-gas flames. These sensors use the ratio of emission intensity in two spectral bands, such as the CH band at 0.43 microns, the C2 band 0.52 microns, and the OH band at 0.31 microns as an indicator of overall fuel/air ratio. These systems are applicable to some, but not all low-pressure combustors, and to high-pressure premixed combustors using natural gas as a fuel. However these spectral bands are not generally applicable in combustors with a high fuel/air ratio in the primary zone or high-pressure, liquid fueled combustors, as emission from soot and soot precursors overwhelm the weaker CH and C2 emissions. Other alternative spectral bands must be used in combustors that produce a large amount of interfering visible emission.
Intensity-based passive optical sensors based on intensity fluctuations are used extensively to monitor the presence of a flame and to sense and control dynamic instabilities at low fuel/air ratio, but the sensed signal is not proportional to the fuel/air ratio over the full range of operating conditions and cannot be used for active control of combustion uniformity. U.S. Pat. No. 5,257,496 teaches the use of intensity-based optical sensors to control uniformity in a turbine engine in a lean-burn state with a specific fuel/air ratio set to produce a combustion temperature of 1700° F., but this technique is not suitable for turbine engines, such as aero-engines, that must operate over a range of fuel/air ratios.
Thus, a need exists for a sensor system that can sense the state of combustion over a range of fuel/air ratios and can be used in conjunction with fuel modulation to actively control combustion uniformity. The sensor should be able to monitor downstream fuel/air ratio from a location adjacent to the fuel nozzle, and should produce an output that is proportional to the state of combustion in the combustor segment influenced by the fuel nozzle.